1. Field of the Invention
This invention is related to material handling equipment and in particular to mechanical devices that restrain a vehicle at a loading dock.
2. Prior Art
Devices that restrain a vehicle against movement away from loading docks are known and widely used. They range from simple wheel chocks to complicated mechanical systems. Mechanical vehicle restraints generally fall into two broad categories, those which restrain by latching on to an abutment of the vehicle, usually the ICC bar and, those which engage a wheel.
Examples of the first type are found in a number of commercial devices such as the Rite-Hite ADL-LDL series and the Serco VR series of vehicle restraints. These devices are covered by U.S. patents, such as U.S. Pat. Nos. 4,264,259; 4,443,150; 5,120,181 and 5,259,718. Such patents are representative of a variety of devices, wall or approach mounted, that move hooks or barriers into position to contact the ICC bar and thus prevent movement away from the loading dock. Such devices have achieved commercial success and are generally effective. They however suffer from operational limitations because of the differences in location and geometry of the target, i.e. the ICC bar. These limitations include a failure to engage in some situations, trailer creep and locking up thus preventing the truck from moving out once the loading operation is complete.
Examples of the second type (wheel chocks) are found in U.S. Pat. Nos. 2,661,505; 3,305,049; 4,207,019; 4,969,792; 5,249,905 and 5,375,965 which show chocking devices which store in the driveway and move toward the wheel to engage and hold the vehicle. These patents represent techniques to automatically place a chock in front of a wheel to prevent a vehicle from moving away from a loading dock, but most have significant limitations. The device shown by Willey in U.S. Pat. No. 3,305,049 is simple in concept, but the screw element that is employed is exposed to impact and could not withstand the lateral and bending forces exerted by the block.
The device shown by Cone in U.S. Pat. No. 4,207,019 occupies space under the rear of the vehicle and could prevent the lowering of a hydraulic tail gate. The device shown by Warner et al in U.S. Pat. No. 5,249,905 provides effective wheel chocking but requires expensive excavation for installation. Also, proper drainage must be provided and heating elements may be required in colder climates.
In U.S. Pat. No. 5,375,965 the chock is placed at the end of a track and the vehicle drives over it. The chock then moves inward toward the wheel until contact is made and then the restraining device is raised to provide a barrier. While eliminating the need for a trench, the device may be damaged as the vehicle rides over it both on entering the dock and leaving. Additionally, the operation requires moving a pair of heavy and complex chock elements on the approach, at positions spaced from the track which may damage the approach itself due to repeated travel, the uneven surface and the presence of obstacles in the path.
Some of these devices have also achieved commercial use but still have serious technical/operational problems. The advantages of this class of vehicle restraint however are that positive engagement with a wheel occurs thus insuring reliability and minimum trailer creep. Nevertheless, these devices tend to be more complex than those restraints which engage the ICC bar and, in some situations require expensive installations such as trenching and the like. Also, they are susceptible to misposition relative to the wheel if the vehicle is not aligned on the approach correctly, i.e. off center or angled.
Additionally, these devices, prior to this invention, approach the vehicle from the front of the approach and move then toward the dock face until they engage the vehicle. This causes storage problems and requires a long run of track or drive member to move the chock into position.
Thus, one limitation common to most automatic wheel chocking devices is that they usually travel through a greater distance and therefore require a much longer time to engage than devices which hook on to the ICC bar. The position of the wheels on the rearmost axle relative to the rear of a vehicle can vary by many feet. Therefore the chock must be able to travel through a long distance to ensure that it can engage the wheels of a variety of vehicles. Usually the chock must overcome a relatively low force to travel from the stored position to engage the wheel, and then must exert a significantly higher force against the wheel to secure the vehicle. Similarly, a high force may be required to dislodge the chock from the wheel because it may have become wedged tightly due to the vehicle being forced away from the loading dock during the loading operation. Also, a high force may be required to dislodge the chock from the stored position in winter conditions. The requirement of high force and fast travel speed through a long distance can be met by a powerful drive mechanism. However, this significantly increases the cost of the drive components as well as the cost of electrical wiring to the drive unit.
As a consequence of these diverse requirements there is a need in the art for a wheel chock using a variable force drive to move the device into position and extend and retract the chock that is less complicated and less expensive than prior devices.
There also exists in the art a need for a wheel chock that is rugged and can withstand abuses of daily use and environmental effects of rain, snow and ice.